The present invention relates to optical processing elements and, more particularly, to improved techniques for the design of optical processing elements for use in optical computing devices.
Optical computing devices, also commonly referred to as “opticoanalytical devices,” can be used to analyze and monitor a sample substance in real time. Such optical computing devices will often employ a light source that emits electromagnetic radiation that reflects from or is transmitted through the sample and optically interacts with an optical processing element to determine quantitative and/or qualitative values of one or more physical or chemical properties of the substance being analyzed. The optical processing element may be, for example, an integrated computational element core (“ICE core”). One type of an ICE core is an optical thin film interference device, also known as a multivariate optical element (MOE). Each ICE core can be designed to operate over a continuum of wavelengths in the electromagnetic spectrum from the vacuum-UV to infrared (IR) ranges, or any sub-set of that region. Electromagnetic radiation that optically interacts with the sample substance is changed and processed by the ICE core so as to be measured by a detector. The output of the detector can be correlated to a physical or chemical property of the substance being analyzed.
A traditional ICE core includes first and second pluralities of optical thin film layers consisting of various materials whose index of refraction and size (e.g., thickness) varies between each layer. An ICE core design refers to the substrate, number and thickness of the respective layers of the ICE core, and the complex refractive indices of the layers. The complex refractive index includes both the real and imaginary components of the refractive index. The layers are strategically deposited and sized so as to selectively pass predetermined fractions of electromagnetic radiation at different wavelengths configured to substantially mimic a regression vector corresponding to a particular physical or chemical property of interest of a substance of interest. Accordingly, an ICE core design will exhibit a transmission function that is weighted with respect to wavelength. As a result, the output light intensity from the ICE core conveyed to the detector may be related to the physical or chemical property of interest for the substance.
Historically, ICE cores have been designed by starting with an extremely large set of random designs (random number of layers with random thicknesses), for example 100,000+ designs. The performance of the various designs would then be determined by calculating one or more performance factors, such as, among others, the standard error of calibration (SEC) of each random ICE core design. Each of the 100,000+ designs would then be iteratively optimized by varying the thickness of each layer by small or minute increments to determine whether a positive or negative change in the performance factors (e.g., SEC) resulted. While this optimization process results in optimized ICE core designs, it requires immense computational capacity and time to undertake this task. Moreover, beginning the ICE core design process with a random design can produce several optimized ICE core designs that are substantially identical, thereby resulting in wasted calculation time for non-unique ICE core designs.